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Movement Across Membranes 1 1. Diffusion 2 Diffusion Particles in liquids and gases move in random directions with a certain amount of kinetic energy 3 Diffusion The overall effect of this is that particles become evenly distributed (equilibrium) 4 Diffusion Kinetic Energy can be increased by increasing the temperature 5 Examples from syllabus: • Diffusion of CO2 into stomata of leaves during photosynthesis. • Diffusion of O2 into the alveoli of the lungs • Diffusion of O2 to respiring cells • Diffusion of glucose, amino acids etc. into the villi during digestion. 6 2. Osmosis The cell membrane is partially permeable – it allows small, particles to pass through by simple diffusion 7 The cell membrane is partially permeable – it allows small, uncharged or high energy particles to pass through by simple diffusion 8 • Osmosis is simply a special case of diffusion… • …that applies to WATER only. 9 10 11 12 Water Potential ‘Water potential’ is a measure of the amount of kinetic (movement) energy water molecules have 13 Water Potential Water potential (y) is a measure of the amount of kinetic energy water molecules have 14 Water Potential In pure water, water potential is at its highest… 15 Osmosis definition: “Osmosis is the diffusion of water molecules from areas of high water potential to areas of lower water potential through a partially permeable membrane” 16 Osmosis Osmosis is the diffusion of water molecules from areas of high water potential to areas of lower water potential through a partially permeable membrane. 17 Exam errors: • Forgetting to use the term ‘water potential’ • Saying water ‘osmoses’ (it doesn’t; it diffuses) • Not mentioning ‘selectively permeable • membrane’ Saying water moves ‘from high to low concentration’ ( you can’t concentrate water! It should be high to low water potential) 18 Osmosis and Animal Cells Cells placed in a solution of higher water potential 19 Osmosis and Animal Cells Cells placed in a solution of higher water potential Cell Water bursts enters by (lysis) osmosis 20 Osmosis and Animal Cells Cells placed in a solution of lower water potential 21 Osmosis and Animal Cells Water leaves by osmosis Cells placed in a solution of lower water potential Cell shrivels (crenation) 22 • This is the reason why animals need kidneys… • …to keep the blood • water levels precisely right. This is called OSMOREGULATION. 23 Osmosis and Plant Cells Cells placed in a solution of higher water potential 24 Osmosis and Plant Cells Cells placed in a solution of higher water potential Cell Water becomes enters by TURGID osmosis 25 Turgid cells keep a plant supported upright. 26 Osmosis and Plant Cells Cells placed in a solution of lower water potential 27 Osmosis and Plant Cells Cells placed in a solution of lower water potential Cell shrivels (PLASMOLYSED) Water leaves by osmosis 28 • When plant cells lose • water and plasmolyse… …the plant is unable to support itself and starts to WILT. 29 Osmosis and Plant Cells And when turgid the cell contents exert outward pressure on the cell wall This helps to support plants and keep them upright. 30 Osmosis and Plant Cells 31 This plant is supported by turgid cells, as well as wood. 32 Examples of osmosis: • Absorbing water in root hair cells of plants. • Absorbing water in the large intestine (colon). • Pallisade cells absorbing water for photosynthesis. • Seeds absorbing water prior to germination. 33 Explain why: • Placing limp lettuce leaves in water for 20 minutes makes them ‘firm’ again. • Pouring salt on slugs effectively kills them. 34 Active Transport Active transport moves substances against a concentration gradient. 35 Active Transport Movement involves the use of ATP from aerobic respiration to supply energy 36 Active Transport Structural changes in the carrier proteins result in the movement of material through the membrane. 37 Examples of active transport: • Uptake of minerals by roots • Concentrating molecules in the vacuoles of plants eg beetroot, rhubarb, mint etc. • Impulses firing in nerves of animals. • Exporting enzymes from cells into the gut. 38